JP2010516043A - Memory device comprising different ferromagnetic material layers and methods of making and using the same - Google Patents

Abstract

A memory element including alternating first and second ferromagnetic material layers is provided.
Each of the first ferromagnetic material layers has a first layer thickness (L ₁ ) and a first limiting current density (JC ₁ ), and each of the second ferromagnetic material layers is a second layer. It has a thickness (L ₂ ) and a second critical current density (JC ₂ ), JC ₁ <JC ₂ , L ₁ is greater than about 300 nm, and L ₂ is in the range of about 20 nm to about 200 nm. The element includes alternating magnetic domains separated by domain walls and in opposite directions. These magnetic domains and domain walls can be moved across the first and second ferromagnetic material layers by applying a drive current. By utilizing this, data can be stored in the storage element as the position of the magnetic domain and the domain wall.
[Selection] Figure 4

Description

Cross-reference of related applications This application is a co-pending US patent application, the name “FORMATION OF NANOSTRUCTURES COMPRISING COMPOSITIONLY MODULATED FERROMAGNETIC LAYERSS BYUSV VERS 19 In relation to (Attorney Docket No. YOR20060194US1-SSMP19770), the application is filed on the same date as this application and is assigned to the same assignee as this application. The entire contents of the above-mentioned co-pending US patent application are hereby incorporated by reference into this application for all purposes.

  The present invention generally relates to memory storage devices. Specifically, the present invention relates to a magnetic memory element that includes alternating layers of different ferromagnetic materials. In the magnetic storage element, data can be stored as the position of the magnetic domain and the magnetic domain wall, and the magnetic domain and the magnetic domain wall can be read or written using the current for reading data from or writing to the storage element. A writing or writing device can be moved through.

  Magnetoresistive random access memory (MRAM) elements store data using the direction of the magnetic moment in the ferromagnetic material. As used herein, the term “ferromagnetic material” refers to any material that exhibits spontaneous magnetization.

  Normally, in a balanced or non-magnetized ferromagnetic material, the magnetic moments of atoms within the relatively large volume of such ferromagnetic materials are aligned parallel to each other by magnetic exchange interactions between the atoms. (I.e., in the same direction) to form a magnetic domain. The magnetic moments of atoms in the same magnetic domain are aligned in parallel, but adjacent magnetic domains are randomly oriented, that is, the magnetic moments of atoms in each adjacent magnetic domain have different directions become. The boundary area between two adjacent magnetic domains is usually referred to as a domain wall. Within the domain wall region, the direction of magnetization gradually changes at the atomic level. However, when a sufficiently strong external magnetic field is applied to a ferromagnetic material, all magnetic domains in the ferromagnetic material are aligned along the direction of the applied magnetic field (ie, the magnetization direction). Even when the external magnetic field is removed, the magnetic domains in the ferromagnetic material remain oriented in the magnetization direction. When another sufficiently strong, new directional magnetic field is applied, the magnetic domains are realigned in the new magnetic field direction.

  One approach to MRAM uses magnetic tunnel junctions as memory cells. A magnetic tunnel junction is formed by separating two layers of ferromagnetic material using a thin layer of insulating material. Each ferromagnetic material layer includes a single magnetic domain. The magnetic domain in the first ferromagnetic material layer has a fixed direction, but the magnetic domain direction in the second ferromagnetic material layer can be changed according to the external magnetic field. As a result, the magnetic domain direction of the second ferromagnetic material layer is either the forward direction or the reverse direction with respect to the direction of the first ferromagnetic material layer, and represents a “0” or “1” state for memory retention.

  However, currently available MRAM devices can only store up to 1 megabit (Mbt), significantly less than the capacity required for most memory applications. In addition, since each MRAM memory cell only stores one bit of data at a time, the maximum possible storage capacity of these elements is greatly limited.

  Accordingly, there is a continuing need for MRAM devices with high storage density and improved large storage capacity.

  US Pat. No. 6,834,005 issued to “SHIFTABLE MAGNETIC SHIFT REGISTER AND METHOD OF USING THE SAME” dated December 21, 2004, owned by the same entity as the present invention. A storage element is disclosed that includes a data storage track formed of a strip of magnetic material.

  FIG. 1A shows a partial view of the ferromagnetic wire 100 described above, which is uniformly magnetized in a single magnetic domain. The arrows in FIG. 1A represent the magnetic moments of the atoms in the ferromagnetic wire 100, which are uniformly oriented to the right. A small section of the ferromagnetic wire 100 can be magnetized to form magnetic domains 102 and 106 in opposite directions as shown in FIG. 1B. The magnetic domains 102 and 106 in the opposite directions are separated from each other by the domain wall 104, and in the domain wall, the magnetization gradually changes from one direction to the other direction as indicated by an arrow in FIG. 1B. Yes. As indicated by a dotted arrow in FIG. 1C, when an electron flow is applied from the right side to the left side of the ferromagnetic wire, the electron group is polarized by the excitation in the magnetic domain 102, so that the right magnetic domain 102 expands. The polarized group of electrons has the same spin as the atoms or ions in the magnetic domain 102 and exerts a force on the domain wall 104. If the density of the drive current is sufficient to overcome the resistance of the ferromagnetic material, the domain wall 104 moves from right to left.

  A typical relationship between domain wall velocity (V) and drive current density is shown in the plot of FIG. When the density of the drive current is lower than the limit current density (JC), the domain wall velocity (V) becomes zero, that is, no domain wall movement is observed. If the drive current is greater than or equal to the limit current density (JC), the domain wall will move at a speed that correlates with the density of the individual drive currents.

  The ferromagnetic wire described above can thus function as a data storage track, in which information can be stored as magnetic domains. Current can be used to move such domains and associated domain walls along the data storage track in the direction of electron flow. As these domains and domain walls move past the reader, information can be read from the data storage track. Similarly, information can be written to the data storage track as these domains and domain walls move past the writing device.

  A storage element disclosed by US Pat. No. 6,834,005 can be used to store a large number of data bits (100 bits or more). This makes it possible to store very large amounts of data using a small number of magnetic elements, making it an important application for various electronic devices such as digital cameras, mobile devices, safety devices, memory sticks, removable storage devices, etc. I'm in charge.

  There is a continuing need for improvements to the memory elements disclosed by US Pat. No. 6,834,005. Specifically, to accurately control the movement of magnetic domains and domain walls along the data storage track, to avoid harmful domain or domain wall drifting, and to enable more accurate and reliable data reading and writing. Is desired. In addition, it would be desirable to provide a more accurate device that can be fabricated at a lower cost.

  The present invention uses a ferromagnetic structure including alternating ferromagnetic layers having different limiting current densities and different layer thicknesses, and accurately controls the movement of magnetic domains and domain walls along the ferromagnetic structure.

In one embodiment, the invention relates to a structure comprising at least a plurality of alternating first and second ferromagnetic layers, each of the first ferromagnetic layers comprising a first layer thickness (L ₁ ) and a first limiting current density. (JC ₁ ), each of the second ferromagnetic layers has a second layer thickness (L ₂ ) and a second limiting current density (JC ₂ ), JC ₁ <JC ₂ , L ₁ is greater than about 300 nm and L ₂ is in the range of about 20 nm to about 200 nm. More desirably, L ₁ is greater than about 400 nm and L ₂ is in the range of about 40 nm to about 200 nm.

  In a preferred embodiment of the present invention, the structure further comprises at least a plurality of alternating magnetic domains of opposite magnetization separated by intervening domain walls. The magnetic domains and domain walls can be moved across the first and second ferromagnetic layers by applying a drive current to the structure.

  The above structure can include one or more additional layers between the first and second ferromagnetic layers. Such additional layers can be extremely thin, for example less than 10 nm. These additional layers, which can be ferromagnetic or non-magnetic, are provided to achieve a variety of different functions, such as separating or insulating the first and second ferromagnetic layers. Can do.

  The first ferromagnetic layer and the second ferromagnetic layer described above are composed of material composition, stress, local roughness, grain size, spin polarization, saturation magnetization, spin transfer efficiency, local spin, lattice constant, coercivity, magnetic anomaly. One or more characteristics selected from the group consisting of directionality, exchange coupling energy, domain wall thickness, and magnetostriction can be different.

  In a preferred but not essential embodiment of the present application, the first and second ferromagnetic layers have different material compositions. For example, the first ferromagnetic layer and the second ferromagnetic layer can contain different ferromagnetic elements. Alternatively, the first ferromagnetic layer and the second ferromagnetic layer can include the same ferromagnetic element mixed or alloyed with different nonmagnetic elements. Furthermore, the first ferromagnetic layer and the second ferromagnetic layer can include the same ferromagnetic element group in different ratios. Furthermore, the first ferromagnetic layer and the second ferromagnetic layer can include those having the same composition of the same element but having different grain structures or grain sizes.

  For example, both the first and second ferromagnetic layers contain Ni—Fe alloys, but the weight ratio of Ni and Fe is different. The first ferromagnetic material layer includes a first Ni—Fe alloy having about 75 wt% to about 85 wt% Ni and about 15 wt% to about 25 wt% Fe, and the second ferromagnetic material layer includes A second Ni—Fe alloy having from about 30 wt% to about 60 wt% Ni and from about 40 wt% to about 70 wt% Fe can be included.

  These first and second ferromagnetic material layers are desirably formed in the form of metal wires having a diameter in the range of about 20 nm to about 500 nm, and more preferably in the range of about 20 nm to about 200 nm.

In another aspect, the invention provides a storage element comprising the above structure, a read element disposed proximate to the storage element to selectively read data from the storage element, and selectively writing data to the storage element. A write element disposed proximate to the storage element to
The data is stored in the storage element as the magnetic domain and domain wall position.

Preferably, the memory element further includes a current source that applies a drive current to the memory element to move the domain wall across the first and second ferromagnetic layers. The drive current is preferably, but not necessarily, a pulsed current having alternating high and low current pulses. The low current pulse desirably moves the domain wall beyond the first ferromagnetic layer and stops at the second ferromagnetic layer, so that a relatively low current density (J _low) that is higher than JC ₁ but lower than JC _2. ). The high current pulse desirably has a relatively high current density (J _high ) higher than JC ₂ to move the domain wall out of the second ferromagnetic layer.

The duration of the high and low current pulses is well controlled to ensure that the domain wall is correctly positioned in the second ferromagnetic layer. More specifically, the domain wall has a first velocity (V ₁ ) under a low current pulse in the first ferromagnetic layer and a second velocity (V ₁ ) under a high current pulse in the first ferromagnetic layer. V ₁ ′) and a third velocity (V ₂ ) under a high current pulse in the second ferromagnetic layer, the low current pulse sustain time (D _low ) is preferably L ₁ / V ₁ or more but shorter than 2 × L ₁ / V ₁ , and the high current pulse sustain time (D _high ) is L ₂ / V ₂ or more but from L ₂ / V ₂ + L ₁ / V ₁ ′ Is short.

  The aforementioned read and write elements desirably read from or write to the storage element at the end of the low current pulse before the next high current pulse is applied.

  In yet a further aspect, the invention comprises the steps of forming a storage element comprising the aforementioned structure, selectively reading data from the storage element, and selectively writing data to the storage element. With respect to the methods involved, data is stored in the storage element as specific locations of magnetic domains and domain walls.

In yet another aspect, the invention provides a step of forming a storage element that includes at least a plurality of alternating first and second ferromagnetic layers, and applying a drive current to the storage element to read the magnetic domains and domain walls. Or a method comprising: moving the write element past a write element; and selectively reading or writing data from the storage element when the domain wall velocity is equal to V _B , , Each of the first ferromagnetic layers has a first limiting current density (JC ₁ ), each of the second ferromagnetic layers has a second limiting current density (JC ₂ ), and JC ₁ <JC ₂ The storage element further includes a plurality of alternating magnetic domains separated from each other by a domain wall positioned therebetween, and the data is stored in the storage element as a specific position of the domain and domain wall to drive current It is, from JC ₂ Big has a constant current density (JC _constant), the domain wall has a first velocity in a first ferromagnetic layer (V _A) at the drive current, the second in the second ferromagnetic layer in said drive current It has a velocity (V _B ) and V _B < _VA .

  Other aspects, features, and advantages of the present invention will become more fully apparent from the ensuing disclosure and appended claims.

FIG. 1A shows a partial view of a prior art ferromagnetic wire that has only one magnetic domain and is uniformly magnetized. FIG. 1B shows a partial view after section magnetization of the prior art ferromagnetic wire of FIG. 1A, which includes at least two oppositely oriented magnetic domains with domain walls in between. FIG. 1C shows the movement of the magnetic domains in the prior art ferromagnetic wire of FIG. 1B upon application of a drive current. 3 is a graph plotting the domain wall velocity of a prior art ferromagnetic material as a function of applied drive current density. FIG. 3 is a graph plotting the domain wall velocity of two different ferromagnetic materials as a function of applied drive current density. FIG. 4A is a partial view of a ferromagnetic wire including two alternating layers of ferromagnetic material according to one embodiment of the present invention, the ferromagnetic wire having a single domain and uniform Is magnetized. FIG. 4B shows a partial view of the ferromagnetic wire of FIG. 4A after segmental magnetization, the wire including at least two magnetic domains in opposite directions with a domain wall in between. FIG. 4C shows the movement of the magnetic domains in the ferromagnetic wire of FIG. 4B by applying a relatively low drive current. FIG. 4D shows magnetic domain movement in the ferromagnetic wire of FIG. 4B upon application of a relatively high drive current. FIG. 4 illustrates an exemplary storage element including alternating layers of different ferromagnetic materials, according to one embodiment of the present invention. FIG. 6 illustrates an exemplary pulse current that can be used to accurately control the domain wall movement in the storage element of FIG. 5, according to one embodiment of the present invention. 7A-7B illustrate a ferromagnetic wire including alternating layers of Ni ₄₅ Fe ₅₅ and Ni ₈₀ Fe ₂₀ according to one embodiment of the present invention, but for imaging purposes, a Ni ₄₅ Fe ₅₅ layer. Is selectively etched. Fig. 4 is a graph showing different domain wall velocities with different ferromagnetic materials under a constant drive current. 6 is a graph showing the time course of domain wall velocity in a memory element including alternating layers of different ferromagnetic materials under a constant drive current.

  In the following description, numerous specific details are set forth, such as specific structures, components, materials, dimensions, processing steps, and techniques, in order to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without these specific details. In other instances, well-known structures or process steps have not been described in detail in order to avoid obscuring the present invention.

  Where an element such as a layer, site, or substrate is described as being “on” another element, the element may also be directly between the other elements It will be understood that elements may exist. On the other hand, when an element is described as “directly above” another element, there are no elements interposed between the elements. Also, when an element is described as being “connected” or “coupled” to another element, it is understood that it may be directly connected or coupled, and that there may be intervening elements Will do. On the other hand, when an element is described as being “directly connected” or “directly connected” to another element, there are no intervening elements.

  As used herein, the terms “critical current density” or “JC” are necessary to drive a domain wall located between alternating domains of opposite direction beyond a particular ferromagnetic material. Threshold current density.

  The entire contents of US Pat. No. 6,834,005 issued to “SHIFTABLE MAGNETIC SHIFT REGISTER AND METHOD OF USING THE SAME” dated 21 December 2004 is incorporated herein by reference for all purposes. It is.

  The present invention realizes accurate control of domain wall motion using two or more materials having different inherent characteristics. Specifically, material composition, stress, local roughness, particle size, spin polarization, saturation magnetization, spin transfer efficiency, local spin, lattice constant, coercivity, magnetic anisotropy, exchange coupling energy, domain wall thickness, Any change in material properties of the ferromagnetic layer, such as magnetostriction, will affect the limiting current density and the domain wall motion speed in the ferromagnetic layer.

  As used herein, the term “ferromagnetic layer” or “ferromagnetic layer group” refers to one or more layered structures that exhibit spontaneous magnetization as a whole. The ferromagnetic layer or layer group of the present invention contains at least one ferromagnetic element and may or may not contain other ferromagnetic elements or nonmagnetic elements.

  The critical current density (JC) necessary for causing domain wall motion in the ferromagnetic material is obtained by the following equation.

  In the above equation, S is the local spin of the ferromagnetic material, a is the lattice constant of the ferromagnetic material, K is the magnetic anisotropy viewed from the hard axis of the ferromagnetic material, and λ is the ferromagnetic material The thickness of the domain wall in the material, which depends on the ratio between magnetic exchange coupling and anisotropy of the ferromagnetic material, e is the charge carried by a single electron (constant),

Is a universal constant. Reference may be made to the papers of Genta Tara and HiroShi Kohno in Physical Review Letters, 92 (8) 088601 (2004) .

  From the above, it is clear that the critical current density (JC) required to cause domain wall motion in a ferromagnetic material is directly determined from the material properties of the ferromagnetic material.

  Furthermore, the speed (V) of domain wall movement depends on the material properties of the ferromagnetic material in which the domain wall is located, and the domain wall is moved through the material by an applied drive current according to the following equation:

The above equation of P is the current polarization, i.e. a percentage of electrons polarized by the local magnetization in the applied driving current, mu _B is the Bohr magneton (constant), e is the charge single electron carries ( it is a constant), M _S is the saturation magnetization of the ferromagnetic material, g is the spin transfer efficiency of the ferromagnetic material, J is the current density of the applied driving current. Europhysics Letters, 69 (6), 990 (2005). A. Thiaville, Y. et al. Y. Nakatani, J. et al. J. Miltat, and Y. Reference may be made to a paper by Suzuki (Y. Suzuki) .

  Therefore, both the limiting current density (JC) and the domain wall transport velocity (V) (when the driving current density is higher than the limiting current density) will be different for different ferromagnetic materials. In other words, the resistance to domain wall motion varies with various ferromagnetic materials.

For example, as shown in FIG. 3, if the limiting current densities of two different ferromagnetic materials A and B are JC ₁ and JC ₂ respectively, and JC ₁ <JC ₂ , higher than JC ₁ but JC ₂ When a drive current having a relatively low current density (J _low ) is applied, the domain wall in the first ferromagnetic material A is moved, but the domain wall in the second ferromagnetic material B is not moved. . Instead, when a drive current with a relatively high density (J _high ) exceeding JC ₂ is applied, the domain walls in both the first and second ferromagnetic materials, A and B, are moved. The speed will be different. Typically, as shown in FIG. 3, the domain wall moves at a considerably lower speed in the second ferromagnetic material B having a higher limiting current density as compared to the first ferromagnetic material A.

  Based on the above observations, the present invention provides a ferromagnetic structure comprising ferromagnetic layers of alternating different material properties as described above. Such a ferromagnetic structure can be used to position the domain wall and ensure that the domain wall moves without any drift in very discrete and precise increments or steps.

Specifically, FIG. 4A shows a partial view of a ferromagnetic wire including alternating first and second ferromagnetic materials, layers A and B, 10 and 12. The layer 10 containing the first ferromagnetic material A has a first limiting current density (JC ₁ ) and the layer 12 containing the second ferromagnetic material B has a higher second limiting current density (JC ₂ ). Have. The entire ferromagnetic wire shown in FIG. 4A is uniformly magnetized in a single magnetic domain and is an arrow representing the magnetic moment of an atom or ion contained in the ferromagnetic wire of FIG. It is shown.

  The ferromagnetic wire of FIG. 4A can be magnetized in its subsections to form alternating magnetic domains 22 and 26, as shown in FIG. 4B. The opposite magnetic domains 22 and 26 are separated from each other by a domain wall 24 located between them, and the magnetization gradually changes from one direction to another in the domain wall. One of the magnetic domains 22 extends from one of the layers 10 containing the first ferromagnetic material A to the adjacent layer 12 containing the second ferromagnetic material B. As shown in FIG. 4B, both the immediately following domain wall 24 and the subsequent magnetic domain 26 (opposite magnetization direction) are located in the next layer 10 that also contains the first ferromagnetic material A.

As shown in FIG. 4C, when a relatively low current density drive current (J _low , JC ₁ <J _low <JC ₂ ) is applied to the ferromagnetic wire, the domain wall 24 has a relatively low limit. current density beyond the rear side of the layer 10 including a first ferromagnetic material a (JC ₁₎ will move. Specifically, the domain wall 24 moves along the direction of the drive current, that is, from right to left in the figure, at a domain wall speed determined by J _low and the material characteristics of the first ferromagnetic material A. However, if the domain wall 24 enters the layer 12 containing the second ferromagnetic material B with a relatively high limiting current density (JC ₂ ), at a relatively low current density (J _low ) drive current, the second The domain wall 24 is “positioned” on the layer 12 as shown in FIG. 4C because it is insufficient to overcome the resistance of the ferromagnetic material B and cause the domain wall 24 to exit the layer 12. Therefore, no matter how long a J _low drive current is applied to the ferromagnetic wire, the domain wall 24 remains in the layer 12 of the second ferromagnetic material B.

However, as shown in FIG. 4D, another drive current with a relatively high current density (J _high , JC ₂ <J _high ) is used to overcome the resistance of the second ferromagnetic material B and the domain wall 24 Can be allowed to escape from the layer 12 into the preceding layer 10. Drive current J _high, in a range sufficient to move the magnetic domain wall 24 exits the layers 12, very can be short-term pulse. In the preceding layer 10 again containing the first ferromagnetic material A, the J used previously to move the domain wall 24 along the layer 10 until the next layer 12 of the second ferromagnetic material B is entered. _{A low} drive current is sufficient.

  In this way, the domain wall can be moved in discrete increments or steps across the ferromagnetic wire of FIGS. 4A-4D using a pulsed drive current.

  In order to more effectively position the domain wall and control the domain wall movement more accurately, the thickness of the ferromagnetic material layer (that is, the layer 12) including the second ferromagnetic material B having a relatively high limiting current density. It is important to suppress this. This layer thickness should be as close as possible to the thickness (λ) of the domain wall in the second ferromagnetic material B. Layer 12 desirably has a thickness in the range of about 20 nm to about 200 nm, more desirably in the range of about 40 nm to about 200 nm, and most desirably about 50 nm.

  The ferromagnetic material layer (i.e., layer 10) that includes the first ferromagnetic material A with a relatively low limiting current density can be of any suitable thickness to accommodate the magnetic domains. Desirably, layer 10 has a much greater layer thickness than layer 12 so that the discrete movement increments of the domain wall in layer 10 can be more clearly distinguished from no movement in layer 12. For example, layer 10 desirably has a thickness of about 300 nm, more desirably about 400 nm, and most desirably about 500 nm.

FIG. 5 shows a storage element 30 comprising “U” shaped ferromagnetic wires or strips, comprising alternating ferromagnetic layers 32 and 34 each containing first and second ferromagnetic materials A and B. Show. On the other hand, the layer 32 containing the first ferromagnetic material A having a relatively low limiting current density has a layer thickness L ₁ greater than about 300 nm. On the other hand, the layer 34 comprising the second ferromagnetic material B with a relatively high limiting current density has a layer thickness L _{2 in} the range of about 20 nm to about 200 nm. Segmented magnetization takes place, forming alternating domains of opposite directions (indicated by arrows in the ferromagnetic wire of FIG. 5) separated from each other by domain walls.

  US Pat. No. 6,834,005 is incorporated herein by reference, but as described therein, a drive current is applied to the ferromagnetic wire of FIG. 5 to move the domain wall therein. And the corresponding magnetic domains in such a ferromagnetic wire are moved past the read or write element so that these elements can read or write data from the ferromagnetic wire.

  Although the ferromagnetic wire shown as an example in FIG. 5 has a substantially circular cross-sectional shape, the present invention is not limited to this, and a square, a rectangle, a triangle, a polygon, a semicircle, an ellipse, etc. It is important to note that any ferromagnetic structure having any symmetric or asymmetric cross-sectional shape is also broadly included. Furthermore, the ferromagnetic structure of the present invention can be a solid wire or any tubular structure surrounding a non-magnetic insulating material or resistive semiconductor core.

In a particularly preferred embodiment of the invention, the domain walls and domains are moved along the ferromagnetic wire shown in FIG. 5 using a pulsed current as shown in FIG. Specifically, the pulse current includes alternating high and low current pulses. Each low current pulse has a relatively low current density (J _low ) greater than JC ₁ but less than JC ₂ , and each high current pulse has a relatively high current density greater than JC ₂ ( J _high ). The low current pulse functions so that the domain wall moves beyond the first ferromagnetic layer 32 and stops at the second ferromagnetic layer 34, and the high current pulse causes the domain wall to exit the second ferromagnetic layer 34 and the first It functions to move into the ferromagnetic layer 32.

  Desirably, the high and low current pulses are timed to minimize domain wall drifting (ie, undesired movement beyond the predetermined point) or staggering (ie, premature stop before reaching the predetermined point). The

As shown in FIG. 3, the domain wall has a first velocity (V ₁ ) under a low current pulse in the first ferromagnetic material layer 32 and under a high current pulse in the first ferromagnetic layer, It has a second velocity (V ₁ ′) and a third velocity (V ₂ ) under a high current pulse in the second ferromagnetic material layer 34. The duration (D _low ) of the low current pulse is L ₁ / V ₁ or more but shorter than 2 × L ₁ / V ₁ , and the duration (D _high ) of the high current pulse is L ₂ / V ₂ or more. in it is _{L 2 / V 2 + L 1} / V 1 ' short it is desirable from.

If a _{D low <L 1 / V 1} , the low current pulses, would not able to have a sufficient length of time to move the domain walls through the entire length of the first ferromagnetic material layer 32. As a result, the domain wall stops prematurely in the first ferromagnetic material layer 32 before reaching the second ferromagnetic material layer 34, and staggering occurs.

The application extension of the low current pulse has little or no effect on the domain wall motion. This is because if the domain wall exits the first ferromagnetic material layer 32 and moves into the second ferromagnetic material layer 34, even if a low current pulse is further applied, This is because the domain wall does not move any further. Therefore, D _low can be made larger than L ₁ / V ₁ . However, in order to avoid a significant heat rise of the wire due to current application and to realize a speedy operation of the element, it is desirable to suppress D _low to 2 × L ₁ / V ₁ or less, and 1.5 × L ₁ / V ₁ is suppressed to below more desirable.

Similarly, if D _high <L ₂ / V ₂ , the high current pulse will not have a sufficient length of time for the domain wall to move out of the second ferromagnetic material layer 34 and move. Therefore, the domain wall will stop prematurely in the second ferromagnetic material layer 34 before reaching the first ferromagnetic material layer 32, and staggering will occur. Therefore, it is desirable to use a high current pulse having a duration (D _high ) of L ₂ / V ₂ or higher.

However, if D _high >> L ₂ / V ₂ , the domain wall continues along the ferromagnetic wire even after the domain wall exits the second ferromagnetic material layer 34, and the first and second ferromagnetic material layer groups continue. Will continue to move and will not stop until D _high is completely finished. Therefore, the extended application of the high current pulse causes harmful drifting of the domain wall, and adversely affects the function of the memory element. Thus, it is desirable to use a relatively short high-current pulse having a shorter high-current pulse duration (D _high ) than L ₂ / V ₂ + L ₁ / V ₁ ′, more preferably L ₂ / V _{2 + 1 / 2L 1 / V} 1 ' shorter than, most preferably _{L 2 / V 2 + 1 /} 4L 1 / V 1' a short pulse from the domain walls without causing significant drifting is first ferromagnetic material Use a pulse long enough to move out of the layer.

  At the end of each low current pulse before the start of the next high current pulse, the positions of the magnetic domain and the domain wall remain approximately at the same point. At this point, the read and write elements disclosed in US Pat. No. 6,834,005 can be used to read from or write to the storage elements of FIG.

  The total number of low and high current pulses in the drive current can be varied based on individual domain wall motion requirements. For example, to move six magnetic domains and five domain walls through a read or write element at a predetermined position, drive currents including six low current pulses and five high current pulses as shown in FIG. It is good to use. In another example, a drive current comprising four low current pulses and three high current pulses should be used to move four magnetic domains and three domain walls through a read or write element in place. I will.

  More than two different ferromagnetic layers can be used to form the ferromagnetic structure of the present invention, and very thin (eg, less than 10 nm) additional nonmagnetic layers can be added to the ferromagnetic structure of the present invention. For simplicity, the following description is primarily directed to structures that include only two different ferromagnetic layers, and these simplified descriptions limit the broad scope of the present invention. Should not be interpreted.

The ferromagnetic materials A and B disclosed above are different from other ferromagnetic or nonmagnetic elements in their pure form as long as they have different material properties and different resistance to domain wall movement. Any suitable ferromagnetic element can be included. For example, ferromagnetic materials A and B include one or more groups of ferromagnetic elements including, but not limited to, Fe, Ni, Co, Gd, Dy, Tb, Ho, Er, and mixtures or combinations thereof. Any ferromagnetic material (s) can be included. In addition to these ferromagnetic elements (groups), ferromagnetic materials A and B include, but are not limited to, Ru, Mo, Mn, Cr, Si, Ge, Ga, As, Cu, Re, Rh, Pt, Au Any non-magnetic element (s) can be further included, including B, P, etc. However, such non-magnetic elements (groups) should not affect the ferromagnetic properties of the materials A and B as a whole.

  Ferromagnetic materials A and B include different ferromagnetic elements, or the same ferromagnetic elements but alloyed or mixed with different nonmagnetic elements, or the same ferromagnetic element group but different ratios. Or those having the same ferromagnetic composition but different particle sizes or grain structures can be included. As described above, the differences between the ferromagnetic materials A and B include material composition, stress, local roughness, grain size, spin polarization, saturation magnetization, spin transfer efficiency, local spin, lattice constant, and coercivity. And one or more material property differences selected from the group consisting of magnetic anisotropy, exchange coupling energy, domain wall thickness, magnetostriction, and the like.

  Alternating layers including materials A and B are processed by chemical vapor deposition (CVD) processing, physical vapor deposition (PVD) processing, atomic layer deposition (ALD) processing. It can be easily formed by one or more well-known deposition processes, such as an electrochemical deposition (ECD) process, and an electroless deposition process.

  In an exemplary but not essential embodiment of the invention, the first and second ferromagnetic material layers, A and B, both contain the same group of ferromagnetic elements and are alloyed in different ratios. . These layers A and B can be deposited by the pulse ECD process described in the co-pending U.S. patent application, the name "FORMATION OF NANOTRUCTURES COMPRISING COMPOSITIONLY MODULATED FERROMAGNETIC LAYERS BY PULSED ECD" Assigned to the same assignee as the application and incorporated herein by reference.

For example, different Ni—Fe alloy layers can be formed by applying a pulse plating potential to a plating solution containing one or more species such as Ni ²⁺ , Fe ²⁺ and NaCl. The pulse plating potential includes a high potential pulse and a low potential pulse. During application of the high potential pulse, a relatively small amount of Fe is deposited and a relatively large amount of Ni is deposited. On the other hand, during application of the low potential pulse, a relatively large amount of Fe is deposited and a relatively small amount of Ni is deposited. In this way, alternating layers of NiFe Ni-rich alloy and NiFe Fe-rich alloy are deposited to form a ferromagnetic wire, which includes alternating layers of different NiFe alloys. . The Ni content of the NiFe Ni-rich alloy thus deposited can range from about 75 wt% to about 85 wt%, and the Fe content of the NiFe Ni-rich alloy can range from about 15 wt% to about 85 wt%. The range can be 25 wt%. The Ni content of the NiFe Fe-rich alloy can range from about 30 wt% to about 60 wt%, and the Fe content of the NiFe Fe-rich alloy can range from about 40 wt% to about 70 wt%. be able to.

FIG. 7A shows a SEM picture of a number of ferromagnetic wires including alternating Ni ₄₅ Fe ₅₅ and Ni ₈₀ Fe ₂₀ layers. The Ni ₄₅ Fe ₅₅ layer has been etched to show more clearly the ability to form compositionally modulated ferromagnetic layers. Usually, NiFe alloys with higher Fe content etch faster than NiFe alloys with lower Fe content. FIG. 7B is an enlarged photograph of a portion surrounded by white circles in FIG. 7A. The Ni ₄₅ Fe ₅₅ layer shown in FIG. 7B has a thickness of about 400 nm and the Ni ₈₀ Fe ₂₀ layer has a thickness of about 200 nm. The Ni ₄₅ Fe ₅₅ alloy and _Ni 80 _{Fe 20} alloy, greatly different coercivity, magnetization intensity, magnetostrictive or other properties, or have a plurality of these different characteristics, the ferromagnetic structure of the present invention It is particularly suitable for forming.

  The present invention further contemplates a storage element that is similar to that described above, but includes alternating layers of any suitable thickness. A substantially constant drive current can be applied to such a storage element to move the magnetic domains and domain walls past the read or write element.

Specifically, this drive current has a constant current density (J _constant ) that is greater than the limiting current density of both the first and second ferromagnetic materials A and B (ie, JC ₁ <JC ₂ <JC _constant ). . Under such a driving current, the domain wall moves longitudinally through the first and second ferromagnetic layers of alternating materials A and B, but the speed in each layer is different. That is, as shown in FIG. 8, the domain wall moves at the first speed (V _A ) in the first ferromagnetic material A and at the second speed (V _B ) in the second ferromagnetic material B, and V _B << _VA . For this reason, as shown in FIG. 9, under this constant driving current, the domain wall speed in the entire memory element oscillates between V _A and V _B, and the domain wall moves at a relatively slow speed (V _B ). Data can be easily read and written.

  In the memory element described above, the second ferromagnetic layer is no longer used for setting the domain wall. Instead, the second ferromagnetic layer is now used to slow down the domain wall motion and ensure a read / write time interval. For this reason, in the case of the second ferromagnetic layer of such a storage element, it is necessary to have a sufficient layer thickness in order to take a sufficient time interval for reading and writing information. For example, the second ferromagnetic layer can have a thickness comparable to the first ferromagnetic layer.

  Although the present invention has been described herein with reference to specific embodiments, features, and aspects, the present invention is not limited thereto, and modifications, changes, applications, and It will be recognized that such modifications, changes, applications, and embodiments outside of the description can be considered as within the spirit and scope of the present invention.

Claims (22)

The structure includes at least a plurality of alternating first and second ferromagnetic layers, each of the first ferromagnetic layers having a first layer thickness (L ₁ ) and a first limiting current density (JC ₁ ). a, wherein each of the second ferromagnetic layer, and a second layer thickness and _{(L 2)} second critical current density (JC _2), a JC ₁ <JC _{2, L} 1 is about 300nm Larger, L ₂ is in the range of about 20 nm to about 200 nm.   A plurality of alternating magnetic domains separated from each other by magnetic walls located therebetween and oppositely magnetized, wherein the magnetic domains and the domain walls are applied with a driving current to the structure to apply the first and second strong domains; The structure of claim 1, wherein the structure is movable across the magnetic layer.   The structure of claim 1, further comprising one or more additional layers between the first ferromagnetic layer and the second ferromagnetic layer.   The first ferromagnetic layer and the second ferromagnetic layer include material composition, stress, local roughness, grain size, spin polarization, saturation magnetization, spin transfer efficiency, local spin, lattice constant, coercivity, and magnetic anisotropy. The structure of claim 1, wherein one or more properties selected from the group consisting of properties, exchange coupling energy, domain wall thickness, and magnetostriction are different.   The structure according to claim 4, wherein the first ferromagnetic layer and the second ferromagnetic layer have different material compositions.   The structure according to claim 5, wherein the first ferromagnetic layer and the second ferromagnetic layer contain different ferromagnetic elements.   The structure of claim 5, wherein the first ferromagnetic layer and the second ferromagnetic layer comprise the same ferromagnetic element alloyed with different nonmagnetic elements.   The structure according to claim 5, wherein the first ferromagnetic layer and the second ferromagnetic layer include the same ferromagnetic element group but have different ratios.   Both the first and second ferromagnetic layers comprise a Ni-Fe alloy, the first ferromagnetic layer comprising about 75 wt% to about 85 wt% Ni and about 15 wt% to about 25 wt% Fe. The second ferromagnetic layer includes a second Ni—Fe alloy having about 30 wt% to about 60 wt% Ni and about 40 wt% to about 70 wt% Fe. The structure according to claim 8. The structure of claim 1, wherein L ₁ is greater than about 400 nm and L ₂ is in the range of about 40 nm to about 200 nm.   The structure of claim 1, wherein the first and second ferromagnetic layers form a metal wire or metal strip having a diameter in the range of about 20 nm to about 200 nm. A storage element comprising the structure and domain wall of claim 2;
A read element disposed proximate to the storage element for selectively reading data from the storage element;
A write element disposed proximate to the storage element for selectively writing data into the storage element;
Storage means including:
Data is stored in the storage element as magnetic domain and domain wall positions.
Storage means.   A current source for applying a drive current to the storage element to move a particular magnetic domain past one of the read and write elements so that data can be read or written from the storage element; The storage means according to claim 12, comprising: The drive current is a pulse current having alternating high and low current pulses, the low current pulse moving the magnetic domains and domain walls beyond the first ferromagnetic layer and in the second ferromagnetic layer. In order to stop, it has a relatively low current density (J _low ) that is larger than JC ₁ but smaller than JC ₂ , and the high current pulse moves through the second ferromagnetic layer through the magnetic domain and domain wall. 14. The storage means according to claim 13, wherein the storage means has a relatively high current density (J _high ) greater than JC ₂ . The domain wall has a first velocity (V ₁ ) under the low current pulse in the first ferromagnetic layer and a second velocity (V ₁ ) under the high current pulse in the first ferromagnetic layer. ₁ ') has, said under the high current pulses in the second ferromagnetic layer has a third speed (V _2), the low current pulse duration (D _low) is, L 1 _/ V ₁ or more but shorter than 2 × L ₁ / V _{1, and} the duration (D _high ) of the high current pulse is L ₂ / V ₂ or more but from L ₂ / V ₂ + L ₁ / V ₁ ′ 15. Storage means according to claim 14, wherein is short.   16. Storage means according to claim 15, wherein the read element or write element reads from or writes to the storage element before the end of a low current pulse but before the next high current pulse. Forming a storage element comprising the structure of claim 2;
Selectively reading data from the storage element;
Selectively writing data into the storage element;
A method comprising:
Data is stored as magnetic domain and domain wall positions in the storage element.
Method.   The method of claim 17, wherein the selective reading and writing is performed by applying a drive current to the storage element and moving the magnetic domain past the reading or writing element. The drive current is a pulse current having alternating high and low current pulses, the low current pulse moving the magnetic domain and domain wall beyond the first ferromagnetic layer and stopping at the second ferromagnetic layer Therefore, it has a relatively low current density (J _low ) that is larger than JC ₁ but smaller than JC ₂ , and the high current pulse moves the magnetic domains and domain walls through the second ferromagnetic layer. _19. The method of claim 18, wherein the method has a relatively high current density (J _high ) greater than JC ₂ . The domain wall has a first velocity (V ₁ ) under the low current pulse in the first ferromagnetic material layer and a second velocity under the high current pulse in the first ferromagnetic material layer. (V ₁ ′), having a third velocity (V ₂ ) under the high current pulse in the second ferromagnetic material layer, and the duration (D _low ) of the low current pulse is L ₁ / _{V 1} or in which although shorter than 2 × _{L 1} / _V 1, the high current pulse duration _{(D high) is,} L ₂ / _V 2 equal to or slightly longer, but _L 2 _/ V 2 + _{L 1} / _{V 1} less than 'a method according to claim 19.   21. The method of claim 20, wherein the selective reading or writing is performed at the end of a low current pulse but before the next high current pulse. Forming a memory element including at least a plurality of alternating first and second ferromagnetic layers, each of the first ferromagnetic layers having a first limiting current density (JC ₁ ). , Each of the second ferromagnetic layers has a second limiting current density (JC ₂ ), JC ₁ <JC ₂ , and the storage elements are alternately separated from each other by domain walls located therebetween The step further comprising a plurality of oppositely oriented magnetic domains, wherein data is stored as magnetic domain and domain wall locations in the storage element;
Applying a driving current to the memory element to move the magnetic domain and domain wall through a reading element or a writing element, wherein the driving current has a constant current density (JC _constant ) greater than JC _2. And the domain wall has a first velocity (V _A ) under the drive current in the first ferromagnetic layer and a second velocity (V _A ) under the drive current in the second ferromagnetic layer. _B ), and V _B <V _A , and
Selectively reading or writing data from the storage element when the domain wall velocity is equal to V _B ;
Including methods.

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